that blue pressure cap ..
is my first thought.
They are not that reliable.  One should always carry a spare.

right now, with no further information or test results..
it sounds like ..
either pressure cap is not holding pressure properly .....

or cooling system pressure is abnormally high ( such as combustion gases
getting into cooling system at the heads or head gaskets ).
If you see bubbles btw...
rising to the surface of the coolant in the pressure bottle
................unless you are in the process of just filling the coolant to
start with ..........that is not a good sign.

if you *always* see a small bubble trail there ...
that's really not a good sign.
Have the air space above the coolant in the main pressure bottle sniffed
with a smog sniffer. If there are excessive HC's there ....it's a combustion
gases getting into the coolant issue.
 It should not do what you are descibing at all.
Try the cap first though ..
as it's so easy to do, and they are known to be a bit dodgey at times.

Scott
www.turbovans.com

----- Original Message -----
From: "Tom Rowsell" <86westy@GEOEXPLOITS.ORG>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Monday, August 22, 2011 11:08 AM
Subject: Coolant Strangeness

> This is likely an FAQ...I'm on the road and searching is awkward with
> cell phone.
> It seems when I drive for an hour or so at highway speed, and stop
> abruptly (to get gas for example) coolant rushes from the left tank
> into the overflow one on the right, sometimes overflowing that one
> onto the ground. In one instance the dash light began to flicker and
> when I pulled over, the right hand tank was full, and the left one was
> down 1/2 litre or so. It's not overheating, but temperature rises at
> idle after a highway run.
> 2.1 litre, recent (last 6 months) new rad, water pump. There are no
> evident leaks, and oil on dipstick looks clean.
> Any suggestions on what to check?
> The needle is generally parked right at the center of the LED...
> Thanks
> Tom